Bacteria in both clinical and non-clinical settings are becoming increasingly resistant to conventional antibiotics, and this resistance is becoming a serious clinical and epidemiological problem for human health. In Gram-negative bacteria, resistance to antibiotics often arises from the production by the organism of β-lactamases, especially metallo-β-lactamases (MBL).
MBL are resistance determinants of increasing clinical relevance. In fact, because of their broad range, potent carbapenemase activity and resistance to inhibitors, these enzymes can confer resistance to almost all β-lactam antibiotics.
MBLs were first detected in the mid-1960s as carried by mobile DNA elements in species with only low pathogenic potential. However, genes encoding MBL spread among major Gram-negative bacteria during the 1990s and this has led to a health crisis arising from the international dissemination of carbapenem-resistant Enterobacteriaceae producing the VIM-type and NDM-type metallo-β-lactamases.
Functional features of these Enterobacteriaceae include potent carbapenemase activity and resistance to clinical β-lactamase inhibitors (clavulanate and sulfones). The activity against β-lactams differs between the different metallo-β-lactamases, and substrate specificity might vary from a narrow range (eg, the CphA metallo-β-lactamase of Aeromonas hydrophila), to an extended range (eg, the VIM-type metallo-β-lactamases, which can degrade almost all classes of β-lactams apart from the monobactams).
There are three major structural subclasses of MBL which share substantial internal diversity. Members of the different subclasses differ not only in their high degree of sequence diversity, but also in the structure of their active sites. In enzymes of subclasses B1 and B3, the active site contains two zinc ions; in members of subclass B2, the active site contains only one zinc ion.
Acquired metallo-β-lactamases have been detected in strains of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter baumannii, and other Gram-negative bacteria. Among acquired MBL, almost all the enzymes belong to subclass B1, which indicates an overall higher propensity for members of this subclass to be captured and spread with mobile genetic elements than for members of subclasses B2 and B3.
As an example, the subclass B1 comprises the IMP-type, the VIM-type, and the NDM-type enzymes.
The IMP-type enzymes, including IMP-1, were first detected in Japan in the late 1980s, and have since been reported worldwide in Enterobacteriaceae and in Gram-negative bacteria. The IMP-type enzymes have broad substrate specificity with a high affinity for cephalosporins and carbapenems, but they have little activity against Temocillin.
The VIM-type enzymes, including VIM-2, were first discovered in Europe in the late 1990s and have since been reported worldwide. VIM-type enzymes were initially detected in P. aeruginosa and in other Gram-negative bacteria, but have since emerged in Enterobacteriaceae, and have become a major problem in some settings. More than 20 different VIM allotypes are known, each with a defined geographical distribution except for VIM-1 and VIM-2, which share a broader distribution than the IMP-type enzymes. The VIM-type metallo-β-lactamases show even broader substrate specificities than the IMP-types, being able to hydrolyse 6-α-methoxy-penicillins. Furthermore, the VIM-type enzymes are unique in the metallo-β-lactamases in that they have a high affinity for carbapenems.
New Delhi metallo-β-lactamase 1 (NDM-1) is a novel metallo-β-lactamase identified initially in a patient hospitalized in New Delhi with an infection caused by Klebsiella pneumoniae. Subsequently, organisms in the Enterobacteriaceae family containing this new 3-lactamase have been found widely distributed throughout India, Pakistan, and Bangladesh and are now occurring in the United Kingdom and in many other countries. The New Delhi metallo-β-lactamase 1 (NDM-1) is a polypeptide of 158 amino acids in length (Accession number AB571289) capable of hydrolyzing a wide range of β-lactam antibiotics including penicillins, cephalosporins and carbapenem antibiotics that are a mainstay for the treatment of antibiotic-resistant bacterial infections. Therefore there is an urgent need for identifying new compounds for detecting and/or inhibiting MBL bacteria.